1. Field of the Invention
The present invention relates to an apparatus for and a method of detecting a malfunction of a controlling unit for a petroleum, chemical or atomic power plant which performs an automatic operation using process control.
2. Description of the Related Art
Conventionally, process control is provided in chemical plants, etc., in order to maintain a process system including tower tanks and piping in a predetermined state. In ordinary process control, various phase values such as flow, level, temperature, pressure, etc., in the respective elements of the plant are measured by using many controlling units and flow control valves associated with the elements are adjusted to the corresponding setting values. If a trouble or malfunction occurs in part of such controlled system, it is difficult to control the entire system appropriately.
For example, there can occur a wrong indication or discrepancy between the phase value and indicated value of a controlling unit. The wrong indication does not immediately cause a fatal damage, but the phase value checked with the setting value will be incorrect to thereby make the correct adjustment of the entire system difficult.
Normally, if a setting value is A, then the phase value or indicated value will also be A. If there occurs a wrong indication in a controlling unit and the indicated value A' becomes A+a where a denotes an deviation, the controlled system operates so as to eliminate the deviation to thereby return the indicated value to the setting value A.
Such repeat of the operation to maintain the indicated value mechanical constant can finally lead to the occurrence of a malfunction where the regulator valve is completely closed.
Although the deviation eliminating operation of the controlled system returns the indicated value of the controlling unit to A=A'-a, the actual phase value is A.increment.=A-a, which is maintained with the deviation a and different from the proper setting value A. If such deviation a continues to be maintained, malfunction can spread through the entire system.
In order to avoid such malfunction, the controlling units at the respective elements of the plant, etc., are required to be monitored also during operation of the plant, etc. Monitoring is performed visually by an operator. However, a wrong indication such as that mentioned above is difficult to visually recognize, so that automatic monitoring by a malfunction detector is performed.
To this end, malfunction detectors are used which detect an deviation between a setting value and the actual phase value and giving an alarm when such deviation has exceeded a predetermined threshold.
Since the deviation between the setting value and the phase value is calculated and the deviation is compared with the threshold in the malfunction detector each time the monitoring operation is performed, there is the problem that the deviation can not be detected as a malfunction until it increases to a predetermined value, and that a wrong alarm is given when a great fluctuation occurs.
In order to calculate the deviation, the difference in quantity between the supplied and discharged materials and the difference in quantity between the supplied and discharged heat may be used, but detection of the malfunction would be delayed when a small difference in quantity between the supplied and discharged materials is accumulated as in the level gauges of the tower tanks.
It is an object of the present invention to provide an apparatus for detecting a malfunction in a controlling unit which is capable of detecting a malfunction of the controlling unit rapidly and appropriately.